This invention relates, in general, to microstructures and to a single mask process for fabricating them and more particularly, to microelectromechanical and microoptomechanical structures-and to a single-mask process for fabricating complete structures including released, movable elements and connectors such as pads, runners, electrodes, and the like on a substrate.
Recent developments in micromechanics have successfully led to the fabrication of microactuators utilizing processes which have involved either bulk or surface micromachining. The most popular surface micromachining process has used polysilicon as the structural layer in which the mechanical structures are formed. In a typical polysilicon process, a sacrificial layer is deposited on a silicon substrate prior to the deposition of the polysilicon layer. The mechanical structures are defined in the polysilicon, and then the sacrificial layer is etched partially or completely down to the silicon substrate to free the structures.
The initial research into surface micromachining established the viability of the technology. Moving rotors, gears, accelerometers, and other structures have been fashioned through the use of such a process to permit relative motion between the structures and the substrate. This process relies on chemical vapor deposition (CVD) to form the alternating layers of oxide and polysilicon and provides significant freedom in device design; however, CVD silicon is usually limited to layers no thicker than 1-2 .mu.m, since residual stress in thicker layers overwhelms the structure and causes curling. Thus, although a large variety of layers can be combined to form very complicated structures, each layer is limited in thickness. In addition, the wet chemistry needed to remove the interleaved oxide layers often takes tens of hours of etching to remove, and once released the structures often reattach or stick to the substrate because of static electricity, and this requires elaborate process steps to overcome. The structures made of polysilicon inherently have a crystalline structure which has low breaking strength because of grain sizes, as well as electronic properties which are inferior to single crystal silicon. Furthermore although this technology is well established, it is not easily scaled for the formation of submicron, high aspect ratio mechanical structures.
In bulk micromachining, a silicon substrate is etched and sculpted to leave a structure. This has typically been done using wet chemical etchants such as EDP, KOH, and hydrozine to undercut single crystal silicon structures from a silicon wafer. However, such processes are dependent on the crystal orientation within the silicon substrate, since the chemistry etches as much as ten times faster in some crystallographic planes of silicon than in other planes. Although the shapes can be controlled to some degree by the use of photolithography and by heavy implantation of boron, which acts as an etch stop, it is difficult to control the process and accordingly, the type, shape and size of the structures that can be fabricated with the wet chemical etch techniques are severely limited. In particular, wet etch processes are not applicable to small (micron size) structure definition, because they are not controllable on that scale.
A dry bulk micromachining process which utilizes thermolateral oxidation to completely isolate 0.5 .mu.m wide islands of single crystal silicon is described, for example, in the article entitled "Formation of Submicron Silicon-On-Insulator Structures by Lateral Oxidation of Substrate-Silicon Islands", Journal of Vacuum Science Technology, B 6(1), January/February 1988, pp. 341-344, by S. C. Arney et al. This work led to the development of a reactive ion etching (RIE) process for the fabrication of submicron, single crystal silicon, movable mechanical structures wherein the oxidation-isolation step described in the Arney et al publication was replaced with an SF.sub.6 isotropic release etch. This process, which allowed the release of wider structures, in the range of 1.0 .mu.m, and deeper structures, in the range of 2-4 .mu.m, is described in U.S. patent application Ser. No. 07/821,944, filed Jan. 16, 1992, assigned to the assignee of the present application. As there described, this dry etch process utilizes multiple masks to define structural elements and metal contacts and permitted definition of small, complex structures in single crystal silicon, and was easy to implement. However, the second lithography step was difficult to apply to deeper structures, particularly because of problems in aligning the second mask. Furthermore, that process relied upon the formation of a silicon dioxide layer on a single crystal silicon substrate, but since other materials such as GaAs or SiGe do not generate an oxide layer the way silicon does, the process could not be transferred to such other substrate materials.
In copending U.S. patent application Ser. No. 07/829,348, a process for releasing micromechanical structures in single crystal materials other than silicon is described. This process uses chemically assisted ion beam etching (CAIBE) and/or reactive ion beam etching (RIBE) to make vertical structures on a substrate, and uses reactive ion etching (RIE) to laterally undercut and release the structure. The process utilizes multiple masks, however, and thus encountered similar problems to the silicon process described above in the formation of deeper structures, and in the alignment of the second mask.
The use of single-crystal materials for mechanical structures can be beneficial, since these materials have fewer defects, no grain boundaries and therefore scale to submicron dimensions while retaining their structural and mechanical properties. In addition, the use of single-crystal materials, particularly single crystal silicon and gallium arsenide, to produce mechanical sensors and actuators can facilitate and optimize electronic and photonic system integration. For example, single crystal silicon structures having a very small mass can resonate without failure at 5 MHz for 2 billion cycles with a vibrational amplitude of plus or minus 200 nm. Accordingly, the fabrication of submicron mechanical structures with high aspect ratios would be highly desirable.